The present invention relates to a method and apparatus for treating exhaust gases, and more particularly, to an exhaust gas treating method and apparatus that uses an adsorbent to separate organic gases from exhaust gases produced in factories.
Organic solvents are used to fabricate semiconductor devices. This results in the production of gases, including organic gases, in semiconductor fabrication plants. Accordingly, exhaust gas treating apparatuses are installed in the fabrication plants to separate the organic gases. A typical exhaust gas treating apparatus contains an adsorbent to adsorb the organic gases. The adsorbent is renewed periodically in a desorbing tower of the exhaust gas treating apparatus and recycled.
With reference to FIG. 1A, a prior art exhaust gas treating apparatus is provided with a cylindrical desorbing tower 1 to renew the adsorbent (activated carbon). The activated carbon adsorbs organic gases in an adsorbing tower (not shown) and is then sent to an upper portion of the desorbing tower 1. Afterward, the activated carbon is heated to a predetermined temperature at a heating portion 2. This desorbs the organic gases adsorbed by the adsorbent and renews the activated carbon. The renewal permits the activated carbon to be repeatedly recycled.
Referring to FIG. 1B, the heating portion 2 includes a brass electric heater 4 and a plurality of upper and lower radially extending fins 5. The heater 4 is arranged along the inner wall of the desorbing tower 1 to heat the interior of the desorbing tower 1 with the fins 5. The upper and lower fins 5 are arranged alternately. Thus, the activated carbon passes through the heating portion 2 slowly. The fins 5 contact and heat the passing activated carbon.
In recent years, organic gases having relatively high boiling points (e.g., N-methyl-2-pyrrolidone (NMP), boiling point 202xc2x0 C.) have been used to fabricate semiconductor devices. Hence, the activated carbon is heated to a temperature higher than 200xc2x0 C. in accordance with the boiling points of the organic gases adsorbed.
Since the activated carbon is heated by the heater 4 with the fins 5 in the conventional desorbing tower 1, it is difficult to heat the activated carbon to 200xc2x0 C. or higher.
If the temperature of the heated activated carbon is lower than the boiling point, the adsorbing capacity of the renewed activated carbon is insufficient. This decreases the treating efficiency (organic gas separating efficiency) of the exhaust gas treating apparatus. For example, if the activated carbon is heated to a temperature lower than the boiling point, the specific surface area, which indicates the adsorbing capacity of the activated carbon, may decrease to about 40% of fresh activated carbon. Therefore, the activated carbon must frequently be exchanged in the conventional exhaust gas treating apparatus to maintain the necessary treating efficiency. This increases the operating cost of the exhaust gas treating apparatus.
Further, the heater 4 is located near the outer wall of the desorbing tower 1. This results in a large difference between the temperature at the portions of the fins 5 closer to the outer wall and that at the portions of the fins 5 closer to the center of the desorbing tower 1. Thus, the heating temperature is non-uniform. This makes it difficult to accurately control the temperature of the activated carbon.
In addition, the heater 4 enlarges the desorbing tower 1. The desorbing tower 1 is covered by a heat insulating material, such as glass wool. The heat insulating material is further covered by a sheet of, for example, stainless steel. These covers further enlarge the desorbing tower 1. Thus, the desorbing tower 1 occupies a large amount of space and is expensive.
It is an object of the present invention to provide an exhaust gas treating apparatus that is relatively compact and is highly efficient.
To achieve the above object, the present invention provides a method for treating exhaust gases including the steps of adsorbing impurity gases included in the exhaust gases with an adsorbent, desorbing the impurity gases from the adsorbent in a desorbing tower by heating the adsorbent selectively to a first predetermined temperature and a second predetermined temperature, which is higher than the first predetermined temperature, and recycling the desorbed adsorbent.
Another aspect of the present invention provides an apparatus for treating exhaust gases. The apparatus includes an adsorbing tower containing an adsorbent to adsorb an impurity gas included in the exhaust gases, a desorbing tower for heating the adsorbent to desorb the impurity gas from the adsorbent and renew the adsorbent, a transfer unit for circulating the adsorbent between the adsorbing tower and the desorbing tower, and a controller for controlling the temperature of the adsorbent.
A further aspect of the present invention provides an apparatus for treating exhaust gases including a desorbing tower for heating an adsorbent to desorb impurity gases adsorbed to the adsorbent and renew the adsorbent. The adsorbent falls through the desorbing tower. A helical heater is arranged in the desorbing tower to heat the falling adsorbent.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.